Utilization of electromagnetic energy for material characterization has found wide spread use and has a rich history. U.S. Pat. No. 2,659,860 issued Nov. 17, 1953 to Breazeale, teaches a method to measure the moisture content of bales of material, by directing a 10 GHz microwave beam through the bale and receiving the beam with another antenna on the far side of the bale from the one which generated the signal. The moisture content of the bale is then determined solely from the attenuation of this signal.
U.S. Pat. No. 4,361,801, issued Nov. 30, 1982 to Meyer and Schilz, teaches a sensing technique that requires measurements of both attenuation and the phase delay of propagation in order to calculate the real and the imaginary components of the complex permittivity measurement in order to measure moisture independent of density, utilizing a 9 GHz and higher microwave applicator. The basis for this measurement is the ratio of the real to the imaginary parts of the dielectric constant, followed by the application of a calibration curve for the specific material of the object. U.S. Pat. No. 6,147,503, issued Nov. 14, 2000 to Nelson et al., describes another moisture sensor algorithm that provides a moisture sensor that is independent of density over the narrow range of densities provided by loose seed kernel samples versus tightly packed seed kernel samples. They teach a technique that operates at 11.3 and 18 GHz similarly using both the attenuation and the propagation delay to calculate the complex permittivity of the material to derive an algorithm for the determination of the moisture content of the material. U.S. Pat. No. 6,476,619 issued Nov. 5, 2002 to Moshe et al., describes a microwave cavity perturbation technique for the sensing of moisture and or density in fibrous yarn, slivers or pad material that has a preferred operating range of 7 to 9 GHz. In the perturbation technique the system is setup with a resonant peak in the signal amplitude versus frequency plot and utilizes the frequency change in the location of this peak as the measure of permittivity change thereby providing a measure of the permittivity from which the moisture content can be estimated assuming a constant density of material. U.S. Pat. No. 6,111,415 issued Aug. 29, 2000 to Moshe, describes the use of a frequency modulated digital pulse of very high frequency microwaves for use as a density sensor. The attenuation and time delay of the signal are analyzed and corrected with empirically derived functions so as to calculate the moisture content and density of the material. Other patents by Moshe et al. include U.S. Pat. No. 5,845,529, issued Dec. 8, 1998, and U.S. Pat. No. 6,107,809, issued Aug. 22, 2000, which utilize a ratio of attenuation to phase delay measurement in a manner similar to the Meyer and Schilz U.S. Pat. No. 4,361,801, referenced above. The reccurring theme between all of these patents is that they all use very high microwave frequencies, typically above 7 GHz, and all of them utilize a measure of the attenuation of the signal after it has been transmitted through the material under test as the primary measure of the moisture content. It should be noted that the radar cross-section of the typical metal bale ties is very large at these high microwave frequencies and has been shown to cause significant signal interference at these very high frequencies, thereby rendering all of these frequencies unusable for use in moisture measurement of metal tied cotton bales. Additionally, these patents do not provide a general solution for measuring material property characteristics.
Given the evaluation of material properties is a fundamental operation for the control of processes and scientific investigations, it is not surprising to find these as well as other notable patents all looking to correlate material properties such as, without limitation, moisture and density, to the response of electromagnetic energy. Other notable references are: U.S. Pat. No. 4,135,131 issued Jan. 16, 1979 to Larsen et al; U.S. Pat. No. 5,256,978 issued Oct. 26, 1993 to Rose; U.S. Pat. No. 5,939,888 issued Aug. 17, 1999 to Nelson; U.S. Pat. No. 6,466,168 issued Oct. 15, 2002 to McEwan; U.S. Pat. No. 7,078,913 issued Jul. 18, 2006 to Pelletier; and U.S. Pat. No. 7,254,493 issued Aug. 7, 2007 to Pelletier, the entire contents of each of which are incorporated herein by reference.
Recently, the use of energy-wave based imaging is becoming more prevalent for detection of interior hidden material properties. In applications for detection of hidden moisture, microwave tomography can be used to image a cotton bale, or other material under test (MUT), and then perform an inverse calculation to derive an estimate of the variability of the hidden interior moisture, thereby alerting personnel to damaging levels of unseen moisture before degradation occurs. One impediment to this type of imaging is when near-by reflectors are too close to be filtered by conventional time-gating techniques, the reflectors divert off-axis energy back toward the receiving transducer which then create large deviations in measured signal propagation delays even though the material properties are uniform.
Other notable applications, of interest to science and industry, are for the determination of the time-varying soil moisture profile around a drip-line by means of the high correlation of soil moisture to the electromagnetic electrical permittivity, primarily responsive to the delay that occurs when an RF wave propagates through the soil. One proposed method of measuring this soil-moisture profile is to surround the drip-line with a series of antennas and have each antenna provide a straight path measurement, from transmitter to receiving transducer, of the permittivity of the material. Unfortunately, for many of the special case applications that perform close-proximity material measurements used in process control applications, there are typically strong reflectors in the same vicinity whose proximity is too close to utilize well known time-gating removal techniques for removal of unwanted reflectors from the desired direct-path measurement of propagation delay.
In the above referenced U.S. Pat. No. 6,466,168, McEwan teaches utilization of a time-domain radar system to measure the time-of-flight of an RF burst using differentially configured sampling receivers, so as to indicate antenna-to-antenna time-of-flight range or to indicate material properties. However there is no teaching regarding the RF pulse's relation to the bandwidth and/or to the relative wavelength ratios of the signal and the influence these parameters have on the received signal with respect to the desired direct path signal and the unwanted interfering multi-path signals that are due to scattering off of the local proximity neighboring off-axis reflectors, whereby said reflectors are both hard metallic reflectors as well as soft reflectors created by changes in material permittivities.
To date, significant work has been performed to utilize antenna arrays to infer the location of reflectors, however the prior art does not provide an accurate method utilizing only a single transmit and receiving antenna for measuring the desired direct-path propagation delay without interference, or with negligible interference, from close proximity off-axis neighboring reflectors.